1. Technical Field
The present invention relates to a current addition type digital analog converter and in particular to a digital analog converter having an interface circuit with a subsequent circuit.
2. Related Art
FIG. 8 shows the configuration of a general current addition type digital analog converter (DAC). As shown in FIG. 8, a usual current addition type DAC has a current conversion section 101 for converting inputted digital data to a corresponding output current and a voltage conversion section 102 for converting the output current of the current conversion section 101 to voltage.
The current conversion section 101 includes a reference power source, a plurality of current source circuits connected to the reference power source, and a switching part controlled by inputted digital data. Current is divided for output by the switching part into a first output terminal (output terminal for conversion) 114 and a second output terminal (output terminal for waste) 115.
As for the voltage conversion section 102, in the simplest configuration, it suffices to have a resistance element 121 connected between the first output terminal 114 and the ground and to convert current I+ outputted from the first output terminal 114 to voltage. However, in this case, the bottom of the output voltage becomes 0 V, which causes mismatching with an input range of a subsequent circuit connected subsequently to the DAC. For this reason, there is provided an offset voltage producing circuit composed of a resistance element 122 and a constant current source 123 to match the range of the output voltage with the input range of the subsequent circuit. On the other hand, current I− outputted from the second output terminal 115 is wasted to the ground via a resistance element 125.
However, when the offset voltage producing circuit like this is provided, the following problem will be presented. First, since an offset voltage is produced by the use of the constant current source and the resistance element, the consumption current of the DAC will be increased. Further, the footprint of the interface circuit will be also increased. Still further, since the output voltage is varied by variations in the constant current source and the resistance element, it is necessary to use an element of high accuracy and to make a cut-and-try adjustment.
Moreover, although there is a case where current (waste current) outputted from the second output terminal is utilized for the purpose of improving the frequency characteristics of the DAC, as disclosed in Japanese Unexamined Patent Publication No. 2003-338759, the current is usually wasted without being utilized. In this manner, a sufficient study has never been made so as to reduce consumption current in the conventional DAC.